The widespread success of cardiac transplantation has definitively proven that replacement of the irreparably damaged heart can sustain patients in good health for many years. The use of a transplanted second heart in the "Piggyback" position has further illustrated that functional support of the badly damaged heart is also highly effective. In 1986, more than 1300 patients received heart transplants in the United States but many thousands who could have been saved with heart transplants died because there were not enough available donor organs. Studies indicate that more than 10,000 patients per year would be candidates for heart transplantation if enough organ donors were available, and many more would be candidates for permanent artificial hearts and cardiac assist devices. Some estimates place the numbers of patients per year who could be saved with permanent artificial hearts between 17,000 to 80,000.
The JARVIK 7.RTM..sup.1 total artificial heart is the only permanent artificial heart which has been used in humans through 1987. The average survival of the five patients so treated was in excess of 9 months, with the longest patient surviving nearly two years (620 days). These patients experienced many complications associated with the early use of a new technology; however, the feasibility of long-term survival was established. FNT .sup.1 JARVIK 7.RTM. is a registered trademark of Symbion, Incorporated, Salt Lake City, UT.
The JARVIK 7.RTM. heart has been used much more extensively as a temporary bridge to transplant to permit patients to survive long enough for a donor heart to be obtained. More than 80 patients were implanted with the JARVIK 7.RTM. heart as a bridge during 1985, 1986, and 1987. The procedure has been done at twenty medical centers in five countries with greater than 75% success, defined as the ability to sustain the patient long enough to receive a donor organ. A high percentage of these patients have survived long term, and many are back to work full time.
Thus, it has clearly been demonstrated that artificial hearts can be effectively used on a widespread basis, and that long-term survival can be obtained. However, to date, no artificial heart system has been developed which is truly practical for widespread application in thousands of patients. The problems associated with present devices involve the inability to provide a high quality of life with high mobility, adequate cardiac function for moderate exercise, and freedom from complications, including blood damage, blood clotting, and infection. Additionally, present systems are relatively cumbersome and require a very extensive level of follow-up care, including medical management of the patient and work with the artificial heart drive equipment.
Each of the numerous artificial hearts that has been patented or disclosed in the literature has attempted to solve one or more problems recognized as important for successful function of artificial hearts. Some attempts have been made to develop artificial heart systems which provide a sufficiently acceptable solution to all crucial functional problems and could, therefore, be truly practical. The reversing electro-hydraulic artificial heart disclosed in my U.S. Pat. No. 4,173,796 was one attempt at such a system. Nuclear-powered artificial hearts developed by the Atomic Energy Commission and The National Institutes of Health, as well as electrically-powered left ventricular assist systems, such as the Thermetics System and the Novacor System, developed with N. I. H. support, have been other attempts. However, all these devices have suffered from problems related to system complexity, large size and weight, or difficulty in control modes and certain fundamental characteristics such as the presence of connectors and crevices within the devices where blood clots can form. No truly seamless electrically- or pneumatically-powered artificial heart system has been disclosed in the prior art.
The problem of thrombus formation and the potential for thromboembolism and stroke is not related only to the existence of seams and crevices within the artificial heart itself. It is also related to the materials of which the heart is fabricated and to the artificial heart valves used. These factors dictate the need for anticoagulant drugs which are undesirable although necessary with devices of the type presently used. Prosthetic heart valves may be categorized generally into mechanical and tissue valves. The mechanical valves generally require anticoagulation, and the tissue valves generally do not. Efforts to incorporate tissue valves into artificial hearts have met with varying degrees of success. As these valves are obtained from animals, they are of irregular shape and are difficult to mount within the artificial heart. However, they are highly efficient and function well at high heart rates. The mechanical valves, in general, are more easily mounted into artificial hearts but require anti-coagulation, may cause significant blood damage under certain pumping conditions, and may be the source of the greatest noise within the artificial heart. In some cases, surgical methods have been developed to preserve the natural outflow valves of the patient's heart, which reduces the number of prosthetic valves needed, reduces the risk of blood clots, and also reduces the cost.
Data from the patients sustained long term with the JARVIK 7.RTM. artificial heart indicates that the heart valves (Medtronic-Hall valves) were a major source of blood damage if the heart drive system was set to a high dp/dt value. They were a likely cause of damage to white blood cells and possibly affected the body's immune defenses. The valves were definite sites for thrombus formation (both on the valve stents and in the crevices where they are mounted to tne heart), and were formation of pannus (scar tissue overgrowth) in long-term cases. Selection and appropriate use of prosthetic heart valves with artificial heart systems can determine success or failure of the entire system.
Infection with permanent artificial hearts is another extremely important problem which is directly related to the design of the device and the follow-up care the patient receives. Pneumatically-powered artificial hearts having relatively large air tubes penetrating the skin carry a high risk of infection when used for periods in excess of one month. The percutaneous lead where the tubes penetrate the skin requires special cleansing and hygiene and is certainly a site where bacteria may enter. Although some successful percutaneous lead systems have been developed capable of permitting electric wires to be passed through the skin without infection for periods as long as ten years, no effective systems for preventing infection with large diameter tubes have been developed to date.
There is little doubt that infection is related not only to a direct mechanism of entry of the infecting organisms into the body but also is related to the presence of prosthetic materials within the body and the body's defense mechanisms in the vicinity of these prosthetic materials. For example, prosthetic heart valves may become infected following dental work when bacteria entering the blood stream near the teeth are carried to the heart and infect the artificial valves. Most artificial heart systems are complex devices with a large surface area of foreign materials within the body. When these foreign materials are placed in certain areas susceptible to infection, the artificial heart may become infected. Surgical procedures which do not prevent all bleeding at the time of closing the chest may leave blood clots around the outside of the artificial heart and within the chest cavity that can serve as a source of nutrients to infecting bacteria.
In order to present the lowest risk of infection, artificial heart systems should:
(1) Cause no functional derangements in the immune system via the interaction of mechanical damage to the white blood cells or via other mechanisms.
(2) Permit surgical implantation with minimal bleeding.
(3) Present the minimal surface area of artificial materials to the bloodstream and occupy a minimum volume within the body without excessive surface area.
(4) Be anatomically adapted to fit within the body without causing pressure necrosis of the tissues.
(5) Heal into the body tissues well with close proximity of vascularized tissues in the vicinity of the surface of the artificial heart.
(6) Require no skin penetration unless such a system is a completely effective barrier against the entrance of bacteria. To meet the above criteria, the optimum artificial heart should be capable of implantation within the natural pericardial sac and thus should be as lightweight and compact as possible.
Approximately 80% of the hemodynamic work of the heart is performed by the left ventricle. Furthermore, disease of the right ventricle is far less common than disease of the left. For these and other reasons, many devices have been developed to assist the left ventricle while relying on the function of the natural right ventricle. Some cases have been reported in the literature of patients treated with left ventricular assist devices where there was no right ventricular function, and the patients survived, but in many cases with temporary assist, left ventricular support only is insufficient, and patients may die from right heart failure if no right ventricular support is provided. However, some surgical procedures have been developed to permit long survival in patients with congenital heart malformations where they essentially have no right ventricular function. Blood entering the right atrium is shunted directly into the pulmonary artery, and patients survive long term with no right ventricular pumping. Most left ventricular assist systems designed for permanent implantation have attempted to augment the pumping capability of the left ventricle and not replace it entirely. This has caused a number of significant constraints on the design of the systems and has often complicated control, requiring synchronization with the natural heart.
Frazier et al. (Journal of Heart Transplantation. Volume 5, Number 4, July/August 1986) have reported a method of replacing the left ventricle with an artificial ventricle while leaving the natural right ventricle in place. In this method, the left ventricular muscle is cut away from the heart, leaving the intraventricular septum in place. Thereafter, the artificial heart is connected to the atrium and aorta. The artificial ventricles used include two prosthetic heart valves and, with the exception of leaving the natural right ventricle in place, the surgical implantation technique is very similar to the techniques widely used to implant the left ventricle in the case of total artificial heart surgery. However, the concept of retaining right heart function and substituting an artificial ventricle for the natural ventricle on the left side has merit. Among the disadvantages of Frazier's method are the lack of mechanical support for the septum which distorts the natural anatomic relationship of the right heart thereby reducing its pumping efficiency, and disruption of the blood supply to the heart muscle which reduces its effectiveness.
Some efforts have been made to replace the function of the left myocardium including the use of mechanical devices implanted in the left ventricular muscle wall or balloons implanted within the cavity of the left ventricle or in the left ventricular muscle wall. One such device is the prosthetic myocardium developed by Dr. Adrian Kantrowitz. A disadvantage of a balloon placed in the left ventricle is that it interferes with the chordae tendineae of the mitral valve. If the natural ventricular muscle is weak, the heart may dilate and effectively act like a large ventricular aneurism, causing a great deal of compliance loss, which reduces the efficiency of the pumping function of the balloon. In effect, air is pumped into the balloon to displace blood by blowing the balloon up, the wall of the ventricle bulges to accommodate the change in volume, and the blood is not actually ejected out of the ventricle through the aortic valve. Thus, balloons in the left ventricle have limited effectiveness. Additionally, a prosthetic myocardium of the type developed by Dr. Kantrowitz can suffer from similar significant compliance losses if the natural tissues of the left ventricle are severely diseased or replaced by scar tissue.
The present invention places specially-designed blood pumps within the natural left ventricular cavity and a prosthetic valve, preferably a tissue valve, is implanted in the natural heart to replace the mitral valve using surgical techniques similar to conventional valve replacement. The patient's aortic valve may be preserved in some individuals or may also be replaced with a prosthetic valve. The blood pumps are very compact in order to fit within the cavity of the natural left ventricle. Pulsatile pumps operating at relatively high heart rates with a relatively small stroke volume are utilized. Rotary hydrodynamic blood pumps may also be utilized, in which case both the mitral and the aortic valves are excised. The artificial heart is surrounded by the natural left ventricular tissues, which remain within the pericardium. The natural coronary artery system is not disrupted, and therefore blood continues to flow to the natural myocardial tissues, which continue to beat. Right heart function is therefore preserved, and the septal geometry is relatively normal. The myocardial tissues which surround the artificial intraventricular blood pump provide a vascular supply to the outside of the artificial ventricle, permitting tissue ingrowth and a stable junction, which is protected from infection by the body's defense mechanisms provided via the vascular supply. The surface area of the blood-contacting membranes is minimized and can be made entirely seamless if the natural tissues of the heart are utilized to support the prosthetic heart valves that are not part of the artificial heart itself.
Table 1 compares heart transplants with ten types of permanent circulatory support systems. These general categories encompass most, but not all, of the possible permanent circulatory support systems. In each category, either left ventricular or biventricular systems are possible, and they are considered to be generally similar. The various systems are compared according to their availability, hemodynamic function, risk of thrombus, system complexity and reliability, infection or rejection risk, quality of life, and cost. The systems have been subjectively scored on a scale from 0 to 4, in the experience and judgment of the inventor, Dr. Jarvik, recognizing that somewhat different scores might be assigned by other experts in the field. These scores are based on reasonable expectations for the optimized devices of each category following major research and developments efforts. Heart transplant, nuclear-powered artificial hearts, intrathoracic electrical artificial hearts both transcutaneously and percutaneously powered, and intrathoracic pneumatic artificial hearts have all been extensively studied and reported in the literature. The scores assigned to the intraventricular designs of the present invention represent the realistic potential in the judgment of the inventor. This numerical comparison is given to emphasize areas of strengths and weaknesses in the varying existing designs. For any permanent circulatory support system to succeed, it must be acceptable in all of the mentioned categories. A score of 0, which represents the identification of an unacceptable weak point in the system, implies that the system will not be viable under present medical and socio-economic conditions. For example, the risk of infection for the pneumatic intrathoracic artificial hearts, which have large tubes penetrating the skin, has been assessed as unacceptable. Although patients' lives may be sustained for more than a year with such devices, ultimately the great majority of these hearts are expected to become infected and cause the deaths of the patients. It is not impossible that effective percutaneous infection barriers for large tubes could be developed, but at the present time, this appears unlikely.
A review of the comparative assessments given in table 1 shows that intraventricular artificial hearts may have the potential to surpass the performance of heart transplants and certainly have the potential to be far superior to present-day pneumatic and nuclear-powered artificial hearts. Intrathoracic transcutaneously-powered electric artificial hearts and intrathoracic muscle-powered devices also show reasonable potential for practical long-term success.
The preferred embodiment is electrically-powered, although intraventricular artificial hearts may be powered by many methods including electro-hydraulic and electro-magnetic actuators.
TABLE 1 __________________________________________________________________________ PERMANENT CIRCULATORY SUPPORT SYSTEMS HEMO- THROM- SYSTEM QUAL- AVAIL- DYNAMIC BUS RELI- INFECTION/ ITY OF ABILITY FUNCTION RISK ABILITY REJECTION LIFE* COST TOTAL __________________________________________________________________________ ALL COMPONENTS IMPLANTED INTERNALLY WITHIN PERICARDIUM I HEART TRANSPLANT 1 4 4 4 3 4 2 22 II INTRAVENTRICULAR 3 2 4 3 4 3 2 21 MUSCLE POWERED WITHIN CHEST, ABDOMEN, ETC. III INTRATHORACIC 3 2 3 3 4 3 2 20 BLOOD PUMP WITH INTRATHORACIC MUSCLE POWER IV INTRAVENTRICULAR 3 2 4 3 3 3 2 20 BLOOD PUMP WITH INTRATHORACIC MUSCLE POWER V INTRATHORACIC 2 3 2 1 3 3 0 14 BLOOD PUMP WITH THORACIC/ ABDOMINAL NUCLEAR POWER BOTH INTERNAL & EXTERNAL COMPONENTS WITH SKIN INTACT VI INTRAVENTRICULAR 4 3 4 3 3 3 3 23 ELECTRIC (TRANSCUTANEOUS) VII INTRATHORACIC 4 3 3 2 3 3 1 19 ELECTRIC (TRANSCUTANEOUS) BOTH INTERNAL & EXTERNAL COMPONENTS WITH SKIN PENETRATION VIII INTRAVENTRICULAR 4 3 4 3 1 3 3 21 ELECTRIC (PERCUTANEOUS) IX INTRATHORACIC 4 3 3 1 1 3 1 16 ELECTRIC (PERCUTANEOUS) X INTRAVENTRICULAR 4 3 4 3 0 1 3 15 PNEUMATIC OR HYDRAULIC XI INTRATHORACIC 4 3 3 2 0 1 1 14 PNEUMATIC OR HYDRAULIC __________________________________________________________________________ KEY 4 EXCELLENT, 3 GOOD, 2 FAIR, 1 POOR, 0 UNACCEPTABLE *Quality of Life Portability, Activities Limits, Maintenance, External Batteries, etc.
Having described the general background and characteristics of intraventricular artificial hearts and methods for their surgical implantation, specific objects of the invention are given in the following section.